FIG. 1 shows the architecture of a typical passive RFID device 2. A powered RFID reader 4 transmits a signal via an antenna 6. The signal is typically 13.56 MHz for MIFARE® and DESFire® systems, manufactured by NXP Semiconductors, but may be 125 kHz for lower frequency PROX® products, manufactured by HID Global Corp. This signal is received by an antenna 8 of the RFID device 2, comprising a tuned coil and capacitor, and then passed to an RFID chip 10. The received signal is rectified by a bridge rectifier 12, and the DC output of the rectifier 12 is provided to a control circuit 14 that controls the messaging from the chip 10.
Data output from the control circuit 14 is connected to a field effect transistor 16 that is connected across the antenna 8. By switching on and off the transistor 16, a signal can be transmitted by the RFID device 2 and decoded by suitable control circuits 18 in the reader 4. This type of signalling is known as backscatter modulation and is characterised by the fact that the reader 4 is used to power the return message to itself.
As an additional security measure, some RFID devices have been adapted to additionally process biometric identification data to provide improved security. In such systems, the user is provided with an RFID card having a biometric template stored on it. A terminal, for example to enable the owner of the card to gain access to money or physical access to a building or office, is provided with a fingerprint sensor and, to authorise the user, a fingerprint read from the terminal is transmitted from the terminal to the RFID card, where a match is performed with the stored template on the card. The RFID card then wirelessly communicates to the terminal the results of the live matching, yes or no.